Add UKM initial_url and session_id fields.

components/ukm/ukm_source.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_UKM_UKM_SOURCE_H_
 #define COMPONENTS_UKM_UKM_SOURCE_H_
 
 #include <stddef.h>
 #include <map>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "url/gurl.h"
 
 namespace ukm {
 
 class Source;
 
 // Contains UKM data for a single navigation entry.
 class UkmSource {
  public:
   UkmSource();
   ~UkmSource();
 
   int32_t id() const { return id_; }
   void set_id(int32_t id) { id_ = id; }
 
+  const GURL& initial_url() const { return initial_url_; }
   const GURL& url() const { return url_; }
+
+  // Sets the URL for this source. Should be invoked when a source is
+  // initialized.
   void set_url(const GURL& url) { url_ = url; }
 
+  // Updates the URL for this source. Must be called after set_url. If a new URL
+  // is passed to UpdateUrl, the initial_url field is populated with the
+  // original URL provided to set_url, and the url field is updated with the
+  // value provided to this method.
+  void UpdateUrl(const GURL& url);
+
   // Serializes the members of the class into the supplied proto.
   void PopulateProto(Source* proto_source) const;
 
  private:
   int32_t id_;
+
+  // The final, canonical URL for this source.
   GURL url_;
 
+  // The initial URL for this source. Only set if different from |url_| (i.e. if
+  // the URL changed over the lifetime of this source).
+  GURL initial_url_;
+
   DISALLOW_COPY_AND_ASSIGN(UkmSource);
 };
 
 }  // namespace ukm
 
 #endif  // COMPONENTS_UKM_UKM_SOURCE_H_